Wavelength division multiplexing ("WDM") has been implemented in high capacity trunk line telephony systems using multi-wavelength all-optical functions. It may likely be the standard in all future trunk services and may ultimately play a role in metropolitan and local-area all-fiber networks. To date, the optical functions employed in such services assumed serial, independent WDM data channels.
Conventional WDM encodes independent base-band digital signals at gigabits/sec [Gb/s] rates onto a series of optical carriers or channels that are evenly spaced in frequency. Single channel rates in non-WDM trunk systems currently run at, e.g., 2.5 Gb/s in continental systems and have reached 5 Gb/s in certain undersea systems. Some trial systems have achieved 10 Gb/s. The first WDM systems will operate with channel rates of 2.5 Gb/s, but may ultimately go higher.
In light of current and continued trends in WDM optical transmission, it is interesting to consider the opportunities to enhance bandwidth, coding, etc., created by WDM. As early as 1988, workers at IBM considered the channel capacity enhancement that could result from a wavelength encoded byte. The IBM workers were able to show that group velocity dispersion-induced bit skew across the wavelength encoded byte severely limits the overall bit rate. In particular, individual channel rates need to be greatly decreased to prevent the individual bits within a given byte from skewing into adjacent time slots occupied by bits at other wavelengths in adjacent bytes. This reduces the gains achieved through wavelength encoding.
Other workers subsequently considered the theoretical possibilities of a spectral or wavelength encoded data bus for improving error correction algorithms and improving data framing as well as the elimination of clock recovery.
In these systems, group velocity dispersion is one limit on transmission capacity. However, dispersion management, not known as a technique even in 1988, is now a well established tool to combat group velocity dispersion in fiber optic systems. Many tools, as well as fiber types, have been developed to ameliorate the bit skew problems noted above. Thus, one important limitation to wavelength encoding of bytes has now been eliminated.
The present invention describes a new approach to implement logic elements which process information that has been encoded in wavelength. The processing occurs entirely in the optical domain and uses ultra-fast wave mixing as a conditional test function. As a result, the clock cycle for the gates described can reach exceedingly high values approaching several hundred Gb/s or even more. The approach enables a modular design similar to that of conventional electronic digital chips. Specific gates (e.g., AND, OR, EXOR, NAND) are programmed into waveguide chips to encode the desired truth tables. The conditions generated by such chips are tested by a four-wave mixing process, creating an output wavelength whose polarization is related to the polarization states of the input waves by way of the truth table.
The idea of using wavelength encoded bytes as a means for introducing one clock cycle error correction into an optical link is one application of the invention. Error correction bits are assigned to some of the wavelengths in the overall byte. A simple (15,11) Hamming code, for example, can be used to assign 4 parity bits to a 15 bit wide wavelength encoded byte (i.e., 11 data bits). If signal-to-noise levels are sufficient to produce 10.sup.-9 error rates without coding, then the simple single error correction Hamming code results in an improvement to 10.sup.-18 error rates. Such a wavelength encoded error correction scheme may be very attractive in situations where burst errors are possible and link integrity must remain high at all times.
Another application is spectral logic or digital circuits employing optical gates. Fiber optic spectral buses may extend the useful range of high speed interconnections between computers without resorting to serial-deserial nodes. Spectral logic can perform certain functions in a word-by-word fashion in this and other applications.
Any application of wavelength encoded bytes benefits from byte processing in the optical domain. Many of the incentives for pursuing wavelength encoded bytes rely upon a certain amount of optical processing. Otherwise, electronic logic operations must be used to code, decode and process information at either end of the link, thereby either greatly slowing down the possible optical channel rates or increasing the required cost. The approach of the present invention allows for construction of any logical function operating at nearly arbitrarily high clock rates. The approach also makes possible modular circuit designs that could ultimately be produced in large quantities to carry out relatively complex calculations.